With photo period is it safe todo 20/4 for veg?

18/6 is the most effective schedule. Cannabis can only grow so much in any 24 hour period and 18 is about the limit. Any more than that and you are getting diminished returns for the electricity consumed. After 18 hours, growth plateaus, meaning you are basically wasting electricity for very little extra growth. Plants do actually need to "sleep" and undertake certain metabolic processes that they can not do during the light cycle, and 4 hours is also not really long enough for these processes to occur. Don't be greedy, treat your plant with love and respect. Plants are living organic beings, not machines to be pushed to their limit, unless you are after "product" and not flowers that will amaze you. Hope this helps,.. Organoman.